Serial Communication Standards


Quite possibly the most widely used communication standard is RS-232 (Recommended Standard 232, also known as Electronic Industries Alliance RS-232-C). This implementation has been defined and revised several times and is often referred to as RS-232 or EIA/TIA 232. The IBM PC computer defined the RS-232 port on a 9-pin D sub connector (DE-9) and subsequently the EIA/TIA approved this implementation as the EIA/TIA 574 standard. The RS-232 and the EIA/TIA-574 specification defines two type of interface circuits: Data Terminal Equipment (DTE) and Data Circuit-Terminating Equipment (DCE). The RS-232 standard is defined as the 9 Position Non-Synchronous Interface between Data Terminal Equipment and Data Circuit-Terminating Equipment employing serial binary data interchange. Both implementations are in widespread use and will be referred to as RS-232 in this document.

RS-232 is capable of operating at data rates up to 20 Kbps at distances less than 15 meters. The absolute maximum data rate may vary due to line conditions and cable lengths. RS-232 often operates at 38.4 Kbps over very short distances.

The voltage levels defined by RS-232 range from -12 to +12 volts. RS-232 is a single ended or unbalanced interface, meaning that a single electrical signal is compared to a common signal (ground) to determine binary logic states. A voltage of +12 volts (usually +3 to +15 volts) represents a binary 0 (space) and -12 volts (-3 to -15 volts) denotes a binary 1 (mark).


The RS-422 specification (ANSI/TIA/EIA-422-B) defines the electrical characteristics of balanced voltage digital interface circuits. RS-422 is a differential interface that defines voltage levels and driver/receiver electrical specifications.

On a differential interface, logic levels are defined by the difference in voltage between a pair of outputs or inputs. In contrast, a single ended interface, for example RS-232, defines the logic levels as the difference in voltage between a single signal and a common ground connection. Differential interfaces are typically more immune to noise or voltage spikes that may occur on the communication lines. Differential interfaces also have greater drive capabilities that allow for longer cable lengths.

RS-422 is rated up to 10 Mbps and can have cabling 1200 meters long at 100 Kbps. RS-422 also defines driver and receiver electrical characteristics that will allow 1 driver and up to 32 receivers on the line at once. RS-422 signal levels range from 0 to +5 volts. RS-422 does not define a physical connector.


RS-485 (TIA/EIA-485-A or RS-485) is backwardly compatible with RS-422; however, it is optimized for party line or multi drop applications. The output of the RS-422/485 driver is capable of being Active (enabled) or Tri State (disabled). This capability allows multiple ports to be connected in a multi drop bus and selectively polled.

RS-485 allows cable lengths up to 1200 meters and data rates up to 10 Mbps. The signal levels for RS-485 are the same as those defined by RS-422. RS-485 has electrical characteristics that allow for 32 drivers and 32 receivers to be connected to one line. This interface is ideal for multi drop or network environments. RS-485 tri state driver (not dual state) will allow the electrical presence of the driver to be removed from the line. Only one driver may be active at a time and the other driver(s) must be tri stated. The output modem control signal RTS controls the state of the driver. Some communication software packages refer to RS-485 as RTS enable or RTS block mode transfer.

RS-485 can be cabled in two ways: two wire or four wire mode. Two wire mode does not allow for full duplex communication and requires that data be transferred in only one direction at a time. For half duplex operation, the two transmit pins should be connected to the two receive pins (Tx+ to Rx+ and Tx- to Rx-). Four wire mode allows full duplex data transfers. RS-485 does not define a connector pin out or a set of modem control signals. RS-485 does not define a physical connector.